Finasteride is a 5α - reductase inhibitor, specifically the type II
and III isoenzymes. By inhibiting 5α - reductase, finasteride
prevents conversion of testosterone to dihydrotestosterone ( DHT )
by the type II and III isoenzymes, resulting in a decrease in serum
DHT levels by about 65–70% and in prostate DHT levels by up to
85–90%, where expression of the type II isoenzyme predominates.
Unlike triple inhibitors of all three isoenzymes of 5α-reductase
like dutasteride which can reduce DHT levels in the entire body by
more than 99%, finasteride does not completely suppress DHT
production because it lacks significant inhibitory effects on the
5α-reductase type I isoenzyme, with 100- fold less affinity for I
as compared to II.

By blocking DHT production, finasteride reduces androgen activity
in the scalp. In the prostate, inhibition of 5α - reductase reduces
prostate volume, which improves BPH and reduces risk of prostate
cancer. Inhibition of 5α - reductase also reduces epididymal
weight, and decreases motility and normal morphology of spermatozoa
in the epididymis. Neurosteroids like 3α - androstanediol and
allopregnanolone activate the GABAA receptor; because finasteride prevents the formation of
neurosteroids, it may contribute to a reduction of GABAA activity ( see also neurosteroidogenesis inhibitor ). Reduction of
GABAA receptor activation by these neurosteroids has been implicated in
depression, anxiety, and sexual dysfunction.

In addition to inhibiting 5α - reductase, finasteride has also been
found to competitively inhibit 5β - reductase ( AKR1D1 ), although
its affinity for the enzyme is substantially less than for 5α -
reductase ( an order of magnitude less than 5α-reductase type I )
and hence is unlikely to be of clinical significance.

Finasteride reduces prostate volume by 20 to 30% in men with benign
prostatic hyperplasia.

Usage

1. Prostate enlargement

Physicians sometimes use finasteride for the treatment of BPH,
informally known as an enlarged prostate. Finasteride may improve
the symptoms associated with BPH such as difficulty urinating,
getting up during the night to urinate, hesitation at the start of
urination, and decreased urinary flow. It provides less symptomatic
relief than alpha -1 blockers such as tamsulosin and symptomatic
relief is slower in onset ( six months or more of treatment with
finasteride may be required to determine the therapeutic results of
treatment ). Symptomatic benefits are mainly seen in those with
prostate volume > 40 cm3. In long - term studies finasteride but not alpha-1 inhibitors
reduce the risk of acute urinary retention ( −57% at 4 years ) and
the need for surgery ( −54% at 4 years ). If the drug is
discontinued, any therapeutic benefits reverse within about 6–8
months.

2. Pattern hair loss

Finasteride is sometimes used to treat pattern hair loss (
androgenetic alopecia ) in men only. Treatment slows further hair
loss and provides about 30% improvement in hair loss after six
months of treatment, with effectiveness usually only persisting as
long as the drug is taken, although on occasion hair loss is slowed
indefinitely following withdrawal. Finasteride has also been tested
for pattern hair loss in women; the results were no better than
placebo.

3. Excessive hair growth

Finasteride has been found to be effective in the treatment of
hirsutism ( excessive facial and/or body hair growth ) in women. In
a study of 89 women with hyperandrogenism due to persistent
adrenarche syndrome, finasteride produced a 93% reduction in facial
hirsutism and a 73% reduction bodily hirsutism after 2 years of
treatment. Other studies using finasteride for hirsutism have also
found it to be clearly effective.

4. Transgender women

Finasteride is sometimes used in hormone replacement therapy for
transgender women due to its antiandrogen properties, in
combination with a form of estrogen. However, little clinical
research of finasteride use for this purpose has been conducted and
evidence of efficacy is limited. Moreover, caution has been
recommended when prescribing finasteride to transgender women, as
finasteride may be associated with side effects such as depression,
anxiety, and suicidal ideation, symptoms that are particularly
prevalent in the transgender population and in others at high risk
already.

Specification

Test Items

Specification

Test Results

Description

White or off-white crystalline powder

Off-white crystalline powder

Specific rotation

-56°~-60°

-58.2°

Identification

IR

Matches with working standard

Complies

HPLC

Solubility

Freely soluble in chloroform and in alcohol;Very slightly soluble in water